They usually go by your LMP, even if it was light. As far as the heartbeat,I think they can pick it up at 12 weeks, or at least that is when they usually listen. Are you getting another U/S? I think the Dr. may be off, he should be going by your LMP as far as the due date. Also, the U/S can be helpful in an EDD.
I would get another opinion ASAP. It can not hurt. Good luck!!